Feedback channel information in communication systems enables the transmitter to avoid interference on the basis of channel conditions. Most adaptive techniques in wireless communications are based on feedback channel information of a certain form.
A representative multiple carrier communication system is illustrated in FIGS. 1 and 2.
FIG. 1 shows a block diagram of a prior-art transmitter 100. Upper layer data is first randomized in a randomizer 101 and then coded in an FEC coder 102. The FEC coder 102 can be of any type of FEC coders. Due to the great error correction performance of CTC (convolutional Turbo codes) and irregular LDPC (low density parity-check codes), these two kinds of channel coding are widely employed. Coded data is subsequently interleaved by a bit-interleaver 103 which can be either a block interleaver or a convolutional interleaver. A modulator 104 maps the interleaved codewords to constellation symbols.
These symbols are mapped by a sub-carrier mapping unit 105 to each sub-carrier according to a predetermined rule. This process can be regarded as mapping data on a time axis to a two-dimensional space of time and frequency. In most systems, logical mapping is different from physical mapping, so as to make sure that adjacent symbols are not mapped to adjacent sub-carriers, to thereby avoid burst fading. The mapped data is then processed by means of a serial/parallel (S/P) converting unit 106, an inversed fast Fourier transformation (IFFT) unit 107, a parallel/serial (P/S) converting unit 108, and a prefix (CP) adding unit 109. These units are basic modules commonly used in the multiple carrier transmission mode orthogonal frequency division multiplexing (OFDM), and are hence not redundantly described here.
FIG. 2 shows a block diagram of a prior-art receiver 200. A received signal is first processed by means of a prefix removing unit 201, a serial/parallel converting unit 202, a fast Fourier transformation (FFT) unit 203, and a parallel/serial converting unit 204. These units are the reversed operation units of the aforementioned units 106, 107, 108 and 109, and are basic demodulating modules of OFDM publicly known to persons skilled in the art, and are therefore not redundantly described here. The function of a sub-carrier de-mapping unit 205 is contrary to the function of the sub-carrier mapping unit 105, as it maps two-dimensional data of time and frequency to a time axis. Constellation symbols are demodulated into bit blocks in a demodulator 206, and then de-interleaved in a de-interleaver 207. The de-interleaved bit blocks (namely FEC coded blocks) are decoded in an FEC decoder 208. The output is subsequently de-randomized in a de-randomizer 209 and provided to the upper layers.
For both convolutional turbo code and low density parity-check code, the codeword length is usually designed to be very long, and this is so because higher randomacity of a long codeword improves the error correction performance.
FIG. 3 is a general view of an encoder 102 with a ½ code rate convolutional turbo code. As shown in FIG. 3, input information bits are divided into 3 streams. The first bit stream goes directly into a multiplexer 305 after having been delayed by a delaying means 301a, and this portion of the codeword is usually called information bits 307. The second bit stream is processed by a ½ constituent convolutional encoder 302a after having been delayed by a delaying means 301b, to obtain the check bits of this constituent encoder. The third bit stream is first interleaved by a CTC interleaver 303, and then encoded by another ½ constituent convolutional encoder 302b to obtain its check bits. The outputs of the encoders 302a and 302b are alternatively chosen by a redundancy deletion matrix unit 304. The outputs of the redundancy deletion matrix unit 304 and the delaying means 301a are multiplexed by the multiplexer 305. As can be seen, convolutional turbo code 306 is composed of information bits 307 and check bits 308. For the convolutional turbo code, the information bits are not performed with any redundancy deletion and protection, which means the information bits part of the codeword plays a more important role than the check bits part, and should hence be additionally protected.
FIG. 4 is a general view of a check matrix of an irregular low density parity-check (LDPC) code. The characteristic of an irregular LDPC code is that the degrees of variable nodes and check nodes are not totally the same. The variable nodes' degree (column weight 404 of a check matrix 401) decides the importance of each bit in a corresponding codeword: greater is more important. That is to say, bits of the part 403 are more important than bits of the part 402 in the codeword.
The CTC code in FIG. 3 and the irregular LDPC code in FIG. 4 share a common characteristic: some bits in a codeword are more important than other bits. We call this kind of FEC code the unequal error protection (UEP) code. Besides the two error correction codes mentioned above, the convolutional code and some linear codes also belong to the UEP codes.
In a multiple carrier wireless communication system, after the data has been encoded based on the UEP code, the bits in one encoded block may be mapped to several sub-carriers after modulation. These sub-carriers usually suffer from fading of differing degrees. The conventional method against fading in the field of encoding is bit-interleaving. All the encoded bits are randomly mapped to different sub-carriers through a bit-interleaver. The probability of deep fading suffered by each bit is the same. In the circumstance the transmitter has grasped the channel fading information through feedback technologies, the conventional transmitter does not provide additional protection for specially important bits.
FIG. 5 illustrates such a circumstance. FIG. 5 is a flowchart showing the channel encoding process at a conventional transmitter side. As shown in FIG. 5, the channel encoding process at a conventional transmitter side includes: forward error correction (FEC) encoding, bit-interleaving, modulating and channel mapping. This process is consistent with the processing flow of the device shown in FIG. 1.
As shown in FIG. 5, UEP channel code (namely the FEC code 505 in FIG. 5) is first inputted in Step 501. The FEC code 505 is an unequal protection (UEP) forward correction code. The length of the code is 36 in the exemplary embodiment. The bits from 9 to 16 and from 29 to 36 are important bits (which are indicated above by the numeral 2, as shown by the reference numeral 510). The bits from 1 to 8 and from 17 to 25 are unimportant bits (which are indicated above by the numeral 1, as shown by the reference numeral 509). The FEC code 505 can be obtained by the FEC unit 102. The FEC code 505 is first interleaved by the bit-interleaver 103 (in Step 502) to obtain a bit-interleaved code 506. The bit-interleaved code 506 is then modulated by the modulator 104 (in Step 503) into 16 QAM constellation symbols 507. After channel mapping (in Step 504) by the sub-carrier mapping unit 105, the constellation symbols 507 are to be transmitted like symbols 508. Reference numerals 511 and 512 show the channel fading intensity (channel state information) of each sub-carrier. Reference numerals 514 and 515 show the importance of each symbol. In the example as shown in FIG. 5, there are altogether 10 important bits that suffer deep fading. As a matter of fact, now that the channel information is known to the transmitting side, there should be corresponding measures to prevent the important bits from suffering deep fading.